Sap. Merry et al., Modulation of quantum yield of primary radical pair formation in photosystem II by site-directed mutagenesis affecting radical cations and anions, BIOCHEM, 37(50), 1998, pp. 17439-17447
Pigment-protein interactions play a significant role in determining the pro
perties of photosynthetic complexes. Site-directed mutants of Synechocystis
PCC 6803 have been prepared which modify the redox potential of the primar
y radical pair anion and cation. In one set of mutants, the environment of
P680, the primary electron donor of Photosystem II, has been modified by al
tering the residue at D1-His198. It has been proposed that this residue is
an axial ligand to the magnesium cation. In the other set, the D1-Gln130 re
sidue, which is thought to interact with the C9-keto group of the pheophyti
n electron acceptor, has been changed. The effect of these mutations is to
alter the free energy of the primary radical pair state, which causes a cha
nge in the equilibrium between excited singlet states and radical pair stat
es. We show that the free energy of the primary radical pair can be increas
ed or decreased by modifications at either the D1-His198 or the D1-Gln130 s
ites. This is demonstrated by using three independent measures of quantum y
ield and equilibrium constant, which exhibit a quantitative correlation. Th
ese data also indicate the presence of a fast nonradiative decay pathway th
at competes with primary charge separation. These results emphasize the sen
sitivity of the primary processes of PS II, to small changes in the free en
ergy of the primary radical pair.